The present invention relates to a pneumatic radial tire adapted to prevent separation on a tread and to eliminate a manufacturing defect in a vulcanizing process. Particularly, the present invention relates to a heavy load pneumatic radial tire such as an Off-the Road tire. The present invention also relates to a method of manufacturing such a pneumatic radial tire.
Conventionally, a tire such as the Off-the Road pneumatic radial tire, which is thick in a tread cross-section thereof, has had a tread rubber member which is thick, wide and long. Thus, a weight of the tread rubber member has been heavy. Moreover, an extruder for the tread rubber member has been limited in terms of equipment and operation. Accordingly, use of the tread rubber member extruded to a required shape has been difficult.
Therefore, a so-called Hot Strip Winding system as one of the tread-forming methods is employed. In this method, a belt-shaped rubber strip having a thickness of about 2 to 3 mm and a width ranging from about 40 to 100 mm is first extruded with the extruder. Next, the rubber strip is spirally wound in a tire circumferential direction while continuously joining side surfaces thereof to each other to form rubber strip layers. Moreover, these rubber strip layers are stacked for a plurality of times so as to have required thickness and shape. Thus, the tread is formed.
Accordingly, on the tread obtained by this method, as shown in FIG. 3, joining boundary lines of the rubber strip are left-side rising in a rubber strip layer 11 which is a first layer counting from a carcass layer side to an outside direction of a tire diameter direction. In a second rubber strip layer 12, the joining boundary lines are right-side rising; in a third rubber strip layer 13, the lines are left-side rising; and in a fourth rubber strip layer 14, the lines are right-side rising. Specifically, lean directions of the joining boundary lines of the rubber strip change alternately in the rubber strip layers. As a result, there remain the following problems {circle around (1)} to {circle around (3)}.
{circle around (1)} For the Off-the Road pneumatic radial tire in which cut resistance performance is strongly demanded, an SBR-series compound is generally used as a tread rubber thereof. Herein, the SBR-series compound is a rubber composition composed of a styrene-butadiene copolymer rubber compounded with a compounding agent such as carbon black. However, adhesive strength between joining boundary surfaces of the rubber strip along joining boundary lines therebetween tends to be weak when the SBR-series compound is used. The reason of the above is as follows. Specifically, in the processes where the rubber strip is extruded, the tread is formed, and then the tread rubber is vulcanized, a part of the joining boundary surfaces of the rubber strip has already been vulcanized due to heat generated in the extrusion process. This leads to insufficient adhesion between the joining boundary surfaces of the rubber strip after the vulcanization process is finished. On the tread, a crack tends to occur along the joining boundary line, if adhesive strength between the joining boundary surfaces of the rubber strip is weak.
{circle around (2)} During contact of the tire, the highest contact pressure is distributed in the region of the tread center. Next, a main deformation distribution in a cross section of a tire meridian direction inside the tread during the contact of the tire is viewed. Then, a deformation component in the tread center region is in a vertical direction since the tread center region is sandwiched among a belt layer and the tread rubbers of both shoulder portions. At this time, the deformation component crosses the joining boundary line of the rubber strip and exhibits a relatively low deformation level. On the other hand, since a side wall closer to the shoulder portion is in a free state, the deformation component leans toward a tire outside direction in the shoulder portion, leading to generation of a main deformation component F exhibiting a high deformation level. Accordingly, in the conventional tread-forming method shown in FIG. 3, the direction of this main deformation component F and the direction of the joining boundary line of the rubber strip become the same in any one of the both shoulder portions. In the shoulder portion where the foregoing two directions become the same, a crack is apt to occur along the joining boundary line, and the growth of the crack is apt to develop.
{circle around (3)} The tread is formed in such a manner that the rubber strip layers are stacked, in which the rubber strip is spirally wound in the tire circumferential direction while continuously joining the side surfaces thereof to each other. Accordingly, unevenness occurs on the tread surface before vulcanization as shown in FIG. 3. Consequently, vulcanization is performed in a state that rubber is not allowed to flow into concave portions because of air remaining therein. Thus, a product external appearance defect (light-tread defect), that a string-like line remains on the tread surface after vulcanization, is apt to occur.
The object of the present invention is to provide a pneumatic radial tire having a tread formed by the Hot Strip Winding system and a method of manufacturing the same, in which separation due to an occurrence of a crack on the tread is prevented, and a manufacturing defect (product external appearance defect) in a vulcanization process is eliminated.
In order to achieve the foregoing object, the present invention provides a pneumatic radial tire having a carcass layer laid between a pair of bead portions and a tread composed of a stacked body of a rubber strip wound continuously and spirally in a tire circumferential direction on the outside of the carcass layer via a belt layer, in which boundary surfaces of the rubber strip of the rubber strip-stacked body forming at least the outermost layers of the both shoulder portions of the tread are leaned respectively toward the outside of a tire diameter direction relative to a tire outside direction in a tire width direction.
The present invention also provides a method of manufacturing a pneumatic radial tire having a carcass layer laid between a pair of bead portions and a tread on the outside of the carcass layer via a belt layer, comprising the steps of forming the tread by stacking rubber strip layers composed of a rubber strip wound spirally in a tire circumferential direction while continuously joining side surfaces thereof to each other, and forming rubber strip layers forming at least the outermost layers of both shoulder portions by winding the rubber strip continuously and spirally from a tread shoulder end to a tread center direction in a tire circumferential direction, while leaning boundary surfaces of the rubber strip respectively toward the outside of a tire diameter direction relative to a tire outside direction in a tire width direction.
As described above, the boundary surfaces of the rubber strip on both the shoulder portions are leaned respectively toward the outside of the tire diameter direction relative to the tire outside direction in the tire width direction. Thus, these boundary surfaces (or joining boundary lines of the rubber strip) cross the direction of the main deformation component in the tread during contact of the tire. Accordingly, the direction of the main deformation component is not given along these boundary surfaces, thus preventing the occurrence and growth of the crack along these boundary surfaces. Consequently, even if mutual adhesion strength of the rubber strip is weak, separation on the tread can be prevented.
Moreover, in the cross section of the tire meridian direction, movement of the tread rubber during vulcanization is the largest on the shoulder portions, and vulcanization is performed in such a manner that the tread rubber flows from the tread center region into the shoulder portions. Particularly, the product external appearance defect on the shoulder portions of the tread surface is as follows.
(a) The boundary surfaces of the rubber strip of the rubber strip-stacked body forming at least the outermost rubber strip layers are leaned respectively toward the outside of the tire diameter direction relative to the tire outside direction of the tire width direction, thus creating the uneven surface. On the uneven surface, the flow of the tread rubber from the tread center region to the shoulder portion during vulcanization becomes gentle, and the flow thereof is not in a disorder. Accordingly, the product external appearance defect does not occur.
(b) On the contrary to the above, the boundary surfaces of the rubber strip of the rubber strip-stacked body forming at least the outermost rubber strip layers are leaned respectively toward the inside of the tire diameter direction relative to the tire outside direction of the tire width direction, thus creating the uneven surface. On the uneven surface, the flow of the tread rubber from the tread center region to the shoulder portion during vulcanization becomes rapid, and the flow thereof is in a disorder. Specifically, the flow of the rubber shows an aspect as if the rubber strip were nappy. Accordingly, the product external appearance defect is apt to occur.
In the present invention, as described above, the boundary surfaces of the rubber strip are leaned toward the outside of the tire diameter direction relative to the tire outside direction of the tire width direction in the both shoulder portions. Thus, the product external appearance defect can be eliminated.